Accelerometers are useful in detecting motion in objects. The theory behind an accelerometer is that motion is detected in a sensitive portion of the accelerometer. This motion is indicative of motion in the larger object or application in which the accelerometer is mounted. Thus, a sensitive accelerometer can quickly detect motion in the application. This quick reaction time may be useful in many ways, such as sensing deceleration in an auto and activating a protective airbag.
Present sensors suffer from many defects, including fatigue of the detecting element of the sensor, high cost, low yield, and complicated manufacturing processes. In one example, U.S. Pat. No. 4,711,128 features a micromachined accelerometer that detects motion of the moving element by a capacitance method. This accelerometer has a very complicated structure made by a micromachining technique, in which two parallel end arms and a plurality of intermediate arms deflect as a result of motion. This type of structure requires a high front-end investment and typically has a low yield in manufacturing because of the many steps involved in manufacture.
A similar technique is used in U.S. Pat. No. 5,610,335, for accelerometers that require even more steps for manufacture. In another example, U.S. Pat. No. 6,497,149 uses a pair of fixed electrodes and a mobile electrode to detect a change in capacitance between the electrodes. The mobile electrode is suspended by springs, and motion is detected in accordance with the mass of the mobile electrode and the spring constant of the springs and the mobile electrode. This construction may suffer from fatigue in the springs, as the mass continuously moves and exercises the springs. If the springs are tempered or the mass adjusted so that fatigue life is enhanced, the sensitivity of the accelerometer may be affected. The present invention is directed at correcting these deficiencies in the prior art.